1. Field of the Invention
The present invention relates to a method of forming a thin film on an end surface of a fiber, such as an optical fiber or a fiber wavelength-conversion element, wherein the fiber consists of a core of an organic crystal and a clad made of glass.
2. Description of Related Art
Conventionally, optical waveguides or wavelength-conversion elements, are made up of a fiber, such as an optical fiber, or a fiber wavelength-conversion element, or the like. These fibers are generally made from a core of an organic crystal and a clad made of glass.
However, conventional fibers which have a core made of an organic crystal, have some problems. These arise because, depending on the kind of the organic crystal, the organic crystal core is sublimated from the end surface of the fiber. This reduces the coupling efficiency of laser light with the fiber. In addition, when the molecules which make up the organic crystal core contain hydrogen bonds, the fibers have displayed poor time-aging stability. This occurs, for example, when the exposed core at the fiber end surfaces contacts water in the atmosphere. The water enters the core and thereby reduces the light transmission properties of the fiber.
As one method of solving the foregoing problems, U.S. Pat. No. 4,830,447 discloses a method where a fluorine containing polymer is dissolved in a solvent and the resulting solution is applied onto a fiber end surface using a spin coater. This forms a thin film on the fiber end surface. However, there has been a problem with this method because the organic crystal core of the fiber is dissolved in the solvent during the spin coating. This results in a reduction in the coupling efficiency of laser light with a fiber treated in this manner.
When a thin film is formed on a fiber end surface using a dry process, such as a vacuum evaporation method, a different problem occurs, sublimation of the organic crystal core. This problem occurs because the time taken to evacuate an apparatus is so long that the organic crystal core is sublimed before a thin film can be formed on the end surface. This can occur even when the organic crystal does not sublime at ordinary temperature and ordinary pressure.